Interaction of a Crack with Crystal Defects

研究了裂纹与位错和间隙杂质等移动缺陷的相互作用，特别关注缺陷重新分布对裂纹扩展的影响。在低于室温的范围内，NaCl晶体的断裂韧性K_<IC>随着温度的升高而逐渐降低。在高于室温的情况下，K_<IC>的值随着温度的升高而增大。计算机模拟表明，在低温下，内部源产生的反屏蔽位错更加活跃，并随着温度的升高而降低 K_<IC> 值，而在高温下，屏蔽位错通过热激活从裂纹尖端大量发射，导致 K_<IC> 值增加。这种趋势与观察到的 K_<IC> 值的依赖性一致，但计算结果和实验结果之间存在一些定量差异。光弹性观察表明，这些差异是由裂纹尖端钝化以及位错结构的稳定引起的。当铁合金晶体中存在可移动间隙杂质时，分布在裂纹尖端前方的杂质会增强局部k场，促进裂纹扩展，从而降低K_<IC>值。杂质原子的四方应变以及裂纹存在下杂质的相互作用增强了正向杂质分布。通过将杂质等离子充电到 Fe-Si 合金晶体中，验证了移动杂质对裂纹扩展的促进作用。

Interactions of a crack with mobile defects such as dislocations and interstitial impurities were investigated with particular attention to the effect of defect redistribution on crack extension. The fracture toughness K_<IC> in NaCl crystals is gradually decreased with increasing temperature in the range below room temperature. At above room temperature, The values of K_<IC> are increased with increasing temperature. Computer aimulation showed that at low temperatures anti-shielding dislocations generated from internal sources are more active and serve to decrease K_<IC> values with increasing temperature, while at high temperatures shielding dislocations are much emitted from a crack tip by thermal activation and cause to increase K_<IC> values. This tendenccy is consistent with the observed dependence of K_<IC> values, but there are some quantitative differences between computed and experimental results. Photo-elastic observations showed that those differences are caused by crack tip blunting as well as the stabilization of dislocation structures. In case of mobile interstitial impurities in iron alloy crystals, the impurities distributed ahead of a crack tip intensify the local k field to promote crack extension so as to decrease K_<IC> values. The forward impurity distribution is enhanced by the tetragonal strain of impurity atoms and also by the mutual interaction of impurities under the presence of a crack. The promotion of crack extension by mobile impurities was verified using the plasma charging of impurities into Fe-Si alloy crystals.